A 20MVA transformer with 30% overload capacity, feeds a 11kV busbar which is connected to multi outgoing feeders.

A 20MVA transformer with 30% overload capacity, feeds a 11kV busbar which is connected to multi outgoing feeders. If a 3 phase fault current of 5000A flows from the transformer to one of the feeders, determine the operating time of the feeder relay R1, the minimum plug setting of the transformer relay R2, and its time setting. Assume a discrimination time margin of 0.5 second and PS in step of 25%.

Rl responsible to clear feeder fault

R2 responsible to clear 11kV bus fault, and to backup all feeder relays

Given:

t = 0.14 / (PSM 0.02 - l) where t is the relay operating time